In stretch-blow molding machines for plastic bottles, prefabricated preforms are conveyed in a conveyor path through a heating station before they are processed in the blowing station. Each preform is already embodied with a mouth which at least essentially corresponds to the mouth of the finished bottle, and it is held in the conveyor path at this mouth by an inserted pre-forming wing mandrel. The pre-forming mandrel is fixed at the mandrel holder mounted in the conveyor path in an exchangeable manner. As many different container types, for example bottles, with various mouths have to be manufactured, the blow molding machine has to be retooled for a changeover to preforms having different mouths. That is to say that the pre-forming mandrel very precisely fits into the mouth and is therefore not suited for different mouth types. In a blow molding machine employed in practice, the coupling consists of a freely ending sleeve at the mandrel holder and a peg at the pre-forming mandrel insertable in the sleeve. In the sleeve, two diametrically opposed, obliquely rising guiding ways with a dead center cam area into which pins arranged at the peg engage are recessed. In addition, a spring is provided which non-positively supports the positive locking. To exchange a pre-forming mandrel, with the conveyor path being stopped, a hand or a tool acts on the pre-forming mandrel, pushes it upwards and then rotates it about its longitudinal axis until the pins exit from the guiding ways. Vice-versa, when a new pre-forming mandrel is inserted, the same is threaded up in the guiding ways with the pins, in the process axially pushed upwards and rotated until it is locked. As considerable thermal stress and vibrations act on the mandrel holder and the pre-forming mandrel during conveyance, the coupling must ensure a secure hold of the pre-forming mandrel and therefore generate a relatively high retention force which has to be overcome when it is exchanged. Handling is troublesome, takes quite a long time and requires considerable skills. As often several hundreds of mandrel holders are arranged in the conveyor path, exchange lasts inappropriately long.